On a ball coupler, there is usually a "jaw" that comes up from behind the ball to secure it in the coupler. It is visible from underneath the coupler.
Get down on the ground, or get a mirror, and look while you operate the latch on top to see what the jaw is doing. Is it moving at all? Is it not moving enough?
BE CAREFUL. Don't put your head or any body part under the tongue of the trailer. It would be just your luck for the coupler to finally release, and drop the hitch on your face.
Rust should not be an issue on a 6 month old trailer unless you towed it through knee-deep salt water. It is likely either a faulty coupler or an adjustment issue.